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<title>avram - a virtual machine code interpreter: 3.1.3 List Transformations</title>

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<a name="List-Transformations"></a>
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<hr size="1">
<a name="List-Transformations-1"></a>
<h3 class="subsection">3.1.3 List Transformations</h3>

<p>Some functions declared in &lsquo;<tt>listfuns.h</tt>&rsquo; are used to implement the
operations described in <a href="List-Functions.html#List-Functions">List Functions</a>. These functions are able
to report error messages in the event of overflow or other exceptional
<a name="index-overflow-3"></a>
<a name="index-exceptions-4"></a>
<a name="index-error-messages-2"></a>
conditions, as described in <a href="Error-Messages.html#Error-Messages">Error Messages</a>. The error messages are
represented as lists and returned to the caller. The occurrence of an
error can be detected by the <code>*<var>fault</var></code> flag being set to a
non-zero value. None of these functions ever causes a program exit except
in the event of an internal error.
</p>
<dl>
<dt><a name="index-avm_005finitialize_005flistfuns"></a><u>Function:</u> void <b>avm_initialize_listfuns</b><i> ()</i></dt>
<dd><p>This has to be called before any of the other functions in this section
is called. It initializes the error message lists, among other things.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fcount_005flistfuns"></a><u>Function:</u> void <b>avm_count_listfuns</b><i> ()</i></dt>
<dd><p>At the end of a run, a call to this function can verify that no
unreclaimed storage attributable to these functions persists. If it
does, a warning is printed to standard error. If <code>avm_count_lists</code>
is also used, it must be called after this function.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005freversal"></a><u>Function:</u> list <b>avm_reversal</b><i> (list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>The reversal of the list is returned by this function if no overflow
occurs. A non-zero <code>*<var>fault</var></code> and an error message are returned
otherwise. The original <code><var>operand</var></code> still exists in its original
order after this function is called. The amount of additional storage
allocated is proportional only to the length of the list, not the size
of its contents.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fdistribution"></a><u>Function:</u> list <b>avm_distribution</b><i> (list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function performs the operation described in <a href="Distribute.html#Distribute">Distribute</a>. The
invalid distribution message is returned in the event of a <code>NULL</code>
operand. Otherwise, the returned value is the distributed list. In any event,
the <code><var>operand</var></code> is unaffected.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fconcatenation"></a><u>Function:</u> list <b>avm_concatenation</b><i> (list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><a name="index-concatenation-2"></a>
<p>The <code><var>operand</var></code> is treated as a pair of lists to be concatenated, with the
left one in the <code>head</code> field and the right one in the <code>tail</code>
field. The invalid concatenation message is returned in the event of a
<code>NULL</code> <code><var>operand</var></code>. The result returned otherwise is the concatenation
of the lists, but the given <code><var>operand</var></code> still exists unchanged.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005ftransposition"></a><u>Function:</u> list <b>avm_transposition</b><i> (list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>The operation performed by this function corresponds to that of
<a href="Transpose.html#Transpose">Transpose</a>. Unlike other functions in this section, the operand
passed to this function is deallocated, and must not be referenced
<a name="index-transpose-2"></a>
thereafter. The transposed list is accessible as the returned value of
this function. If the original <code><var>operand</var></code> is still needed
after a call to <code>avm_transposition</code>, only a copy of it should be
passed to it, obtained from <code>avm_copied</code>. The invalid transpose
error message is the result if the operand does not represent a list of equal
length lists.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fmembership"></a><u>Function:</u> list <b>avm_membership</b><i> (list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function computes the membership predicate described in
<a name="index-member-1"></a>
<a href="Member.html#Member">Member</a>. The operand is a list in which the <code>tail</code> field is
a list that will be searched for the item in the <code>head</code>. If the
item is not found, a <code>NULL</code> list is returned, but otherwise a list
with <code>NULL</code> <code>head</code> and <code>tail</code> fields is returned. If the
operand is <code>NULL</code>, an error message of invalid membership is
returned and <code>*<var>fault</var></code> is set to a non-zero value.
</p>
<p>The <code>avm_membership</code> function calls <code>avm_binary_comparison</code>
in order to compare lists, so the same efficiency and side-effect
considerations are relevant to both (<a href="Comparison.html#Comparison">Comparison</a>). It is not
necessary to <code>#include</code> the header file <code>compare.h</code> or to
call <code>avm_initialize_compare</code> in order to use
<code>avm_membership</code>, because they will be done automatically.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fbinary_005fmembership"></a><u>Function:</u> list <b>avm_binary_membership</b><i> (list <var>operand</var>, list <var>members</var>, int *<var>fault</var>);</i></dt>
<dd><p>This function is the same as <code>avm_membership</code> except that it
allows the element and the set of members to be passed as separate
lists instead of being the head and the tail of the same list.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fmeasurement"></a><u>Function:</u> list <b>avm_measurement</b><i> (list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function implements the operation described in <a href="Weight.html#Weight">Weight</a>, which
pertains to the weight of a tree. The returned value of this function is
a list encoding the weight as a binary number, unless a counter overflow
occurs, in which case it&rsquo;s an error message. As noted previously, the
weight of a tree can easily be exponentially larger than the amount of
<a name="index-native-integer-arithmetic-1"></a>
memory it occupies, but this function uses native integer arithmetic for
performance reasons. Hence, a counter overflow is a real
possibility.
</p></dd></dl>

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